As electronic equipment and/or systems controlled by such equipment become ever more complex so the number of signals required to be transmitted between individual pieces of equipment and/or subsystems increases. There is therefore a corresponding increase in the number of transmission lines, or cables, that are required to be provided between individual items of equipment. In certain applications where physical space is limited, or where it is desirable to minimise the weight of physical transmission lines, or where new systems are being retrofitted into existing installations it becomes impracticable or impossible to provide a wire for each individual signal.
One such area where this conflict arises is on aircraft. Most aircraft have a number of onboard electrical generators that are driven by the aircraft engines to supply the power for the aircraft's electronic and electrical systems. Generally, each generator is controlled by an associated electronic generator control unit (GCU) that ensures the power supply provided by the generator is within certain predetermined parameters e.g. frequency and voltage, that are appropriate for the electrical equipment to be supplied. However, there is clearly a requirement to provide a power supply to the aircraft electrical systems when the aircraft is on the ground and the engines are not in operation or are in a start up or shut down mode. For example, it is often desirable to operate the air conditioning units and lighting on the aircraft when it is on the ground to allow simple maintenance and cleaning operations to be completed, or equally to enable the various flight systems to be operated, such as navigation computers, prior to the departure of the aircraft. It is therefore necessary to provide an auxiliary power supply. The auxiliary power supply may either be a portable generator, for example a diesel powered generator, that is connected to the aircraft's electrical system, or may be an auxiliary power unit mounted on the aircraft itself.
In either case, when switching between the aircraft's power supply and the auxiliary power supply it is undesirable to have any interruption in the power supply to the electrical systems. The seamless switch from one power supply to another is referred to as a “no break power transfer” (NBPT) and it is accomplished by very briefly having both power supplies connected to the aircraft systems at the same time.
Successfully providing NBPT requires careful synchronising of the auxiliary power supply and the aircraft generators to avoid any sudden unequal loading of the electrical systems. This is accomplished by providing a fixed frequency reference signal from a frequency reference unit to the aircraft's generator control units and also providing command signals that are transmitted between the auxiliary power supply control unit and the aircraft generator control units. The frequency reference signal and the command signals have in the past been provided over separate transmission lines or cables. It is not always practicable or possible to provide individual signal lines, especially where equipment is being retrofitted.
It is therefore desirable to provide a method of transmitting more than one signal using the same transmission line.